Noise rejection circuit



F 5, 1960 R R. FRENCH ErA| 2,925,494

' NOISE RBJEQTION cmcun Fi1e d April 17, 1957 TRANSDUCER OUTPUT DIGITS PLAYBACK BISTA BLE CIRCUIT OUTPUT FIGTII FIG; 2

BISTABLE FROM TRANSDUCER OUTPUT INVENTORS ROBERT FRENCH ROBERT E CASEY A NT Robert L. French," Boonto miand 'Rohert Fl Casey, Pompmore closely spaced to obtain a larger storage capacity "between noise and signal is always atfthefoptimum point. fF Ihe ClICHllI Df' tllfiilllfldldt: invention'changesthe levels at whichit discrimina s between 'no'isearid desired signal tionhandled; 'Ihe r elation hetj weenjthej change and ith sr1en that the level -of 'di'scri'rm 4v will-best distinguish between the 'fAn objectfofjtlfl intense Y I "which will adapt automatically to ch ng and noise being 'handled. j f

understanding of the inyentioniniayfbezhadby referring 7' fjunctionwith the ac qmpanyiq drawin'g'sfin which; i a

Fig. '1'*'show's a representative .putpu waveform from I a magnetic. transducer playing sack-one type of rmagne'ti- "Fig. 1," was {recordedby"applying gpu es of one'polarity digits. A's willbc -seen from 'ri-gr rjia" high'amp pulse of one-polarity is" obtained in the playback each v a digit interval'and systems for coordinatingand using; 7 information in this form are well known in the, digital bistable circuit triggered to one condition by the h h dition by the high amplitude negative playbaclr pulses; Wheres'uch an outpursys'temis iusedgthe jd'esired "playback signal to trigger thejbistable circuit consists solely of the highamplitude pulsesma'rking each change of information from a 1 tofa OTdigit and vice versa. 'It will be noted that the, amplitude 'ofthese pulses, as shown in .Fig." 1,increases=as the number of successive; chine o p y n 'P Din opposite value digits immediately preceding the change I j i j increases; A ,hip onthe ,leadin'g edge of the high 'a'rnpp cation April 1'1,"mszsmsmoiissspzs 1 iwd P S increases r v r m lin e j R q 7 j proportion as the pulse. 40mins. c1. 2so-'27 p 2,925,49 V NQI SE'REJEC'I'Ib N cmcnrr. 1

ton Plains, NJ., assignors to Monroe CalculatingfMa- 7 These hipsf on the "leading edge of the high amplitude I pulses can cause premature triggering of the bistable lv v j j j circuit and soproducean erroneousindication of'th'e invention relates to anoiserejection circuit which valu f the recedingdi'git. 'The fhipslincrease in is amplitude discriminative and'wh'ichshiftsthelevels amplitude as the recorded information is 1' more. closely v of discrimination with changes in the charac'teristic' s of a d the ma neticim'edium. The storagecapacity the Signal g n d- V of a givenma'gnetic recording unit-jcan-thusbe increased if' the ability of the playback circuit to distinguishf the In some types of ma 'etic recording systems for digital information, the noise anddesired signal le'velslin the hip s froin the desired; signal is in r a d;

playbackchange with the nature of the" preceding recorded information.- The-magnitude of these changes increases *as the "digits of the recorded information are lncompute'rs and other digital data processing equip- ;rne'nt, a'numbe'r of magnetictran'sducerssensingdiiferent recording channels are selectively switched "to a given p playback circuit as information at ditferent locations in given recording a a 1 l fi wv s '25 storage is required. Differences in losses in the, Selection spacing of digits ispossibleif the'leve'l of discrimination {paths or in the magnetic transducers" and their physical spacing' from; the recording medium result in Vdilferen t 'output signal levels. "The' different signal levels "which 'musf'be' handled malt it more difiicult to d rlmina'te :hetweensigrialandn' n'all cases-f in accordance with'thenature off the-precedinginforma- 3O Refernng- .aL'ma nature of the preceding information 1 h dled received Q 7 alsoconnected through sistor, 1 6 t' o the'l junctionofkresistors', 17 arid"l8l Th other fern "minalfof re's'isto'rv1-7 isflconnectedto. the sou [negative po'tfehtiarw thef'other term nation j 118: is

ound. -Condnsfe 1 9'is conn tdiin 'tconnected fto 'parallel,with resistor'ltii a, p a Tube 291m iits plate .cqn eqed tofthe pq irive 'yoltage: source through resistorf21" and its cathodeto-the negative to: the following'description and claims-taken in' 'con- 45 v lt'zi ggbur gjhrqughth Hand-5 23;. con ed e gofimbajzp aadgthegeominor ermina z r and 1 8. 1 Rec'tifi' 25 ected 'betwe cally recorded digital.information, and,thej col'l' Spbriding output waveform desired from the playback circuit. 7 'Fig. 2 is a schematic'diagram bfQan-embodiment of the t nr 5 I W The binary information, whose playback is shownl in litl'ldeif f connectedto' ground Thefplat eYoft ube 20 is connected to the-"trigger input to bistable circuit 30. I Bistable' cirthe recorded information c n from i a, I 'cuit maybe of any suitable type which issct to'one "0 digit and of the opposite polarity each time it changes: 'condition 5 by a 'positiyegoing voltage and to {11; oth r from a 5'0" to a 1" digit. Each successive' digit {inja (l0g i i by" a e ati e going l series of like digits produces'pulses of; lower amplitude, g [The desired playback bistable circuit output waveform,

shown in Fig. 1; is positive during each fl" digit interval and negative dur'ingca'ch fO? digit' interval. .It is up 7 "1 resentative of the inforrhtion'being played back when the cathode arms: 20, rectifier 26 conducts and the,

' zThe transducer output signal applied-to the grid of {tube l2 rthrough condenser ll appears acrossresistors-14. "and 15 due'to" the cathode follower action. When the junction- 0f resistors 14 and 15' goes more positivethan.

more'positive 'goingpart of'the signal is applied across 5 resistors 22 and 23. aWhen thecathode of tube 12 goes] 1 7 more "negative'thanjthe' junction of resistors 22 and 23,; a 3

computer field. The" waveform can-he produced by a rectifier 25 conducts andthe more negative going coordinated with a series of clock pulses'marking each A of the signalis applied across resistor 23.3 g v v V esplitudtrosifivt Pulse and 9 w e's na w v u l =2 w 9 when and at 5 junction of resistors 14 and 15 may be at substantially the same potentials as the cathode of tube and the. junction of resistors 22 and 23 respectively. Only sigasasnea.

nalcomponen'ts greater in amplitude thanhalf the .voltage acrossresistor 22 Twill then cause rectifiers25 or 26 ponents also appear across plate load resistor 21.

The functioning of the circuit thus far described is substantially the same as that of the amplitude discriminative circuit disclosed in US. patent application Serial Number 539,717 filed October 11, 1955 by Robert F. Casey. Signal'and noise components below the amplitude necessary to affect the cathode of tube 20 are rejected and only those signal components greater than this amplitude appear across resistor 21 and are applied to the trigger input of bistable circuit'30.

In the instant circuit, the grid of tube 20' is coupled to an output of bistable circuit through condenser '27 and the integrating network formed by resistor 28 and condenser 29. Condenser 29 charges through resistor 28 to alternately diiferent'voltages each time bistable circuit 30 changes coinditiona The values of condenser 27 and resistor 24 are such that their time constant is relativelylong and the potential on the grid of tube 20 changes by substantially the same amount as the voltage across condenser 29.

Anincreasein potential on the grid of tube 20 will inore'asethe average potential on its cathode and on the junction of resistors 22. and 23. A smaller negative signalamplitu'd'e will then be requireddto cause rectifier 25 to'con'duct than before the increase. A larger positive signal amplitude will be required to cause, rectifier 26 to conductand the levels of amplitude discrimination will have been-shifted in one direction by the increase of potential onth'e'grid of tube 20. A decrease in potential on thegrid of tube 20 will shift thelevel of amplitude discrimination in the opposite direction.

The. charge on condenser 29 willvary betweenthe twowalues of potential provided by the output'from bistable'circuit 30 in its. two different conditions. The change from one value to the other across condenser 29 will be delayed 'duefto'the time required for charging condenser 29 through resistor 28. This time constant is chosen so that the change of voltage after 'bistablecircuit 30 changes condition substantially follows the same pattern as'the increase of amplitudeof the leading fhips" onthe "high'amp'litude pulses from the transducer.

' The changefin voltage 'on condenser. 29 after each change or condition'of bistable circuit shifts the levels of positive andfn egative"amplitude discrimination through itsaction onjthe grid of tube 20; Thedirection of the shift'is suchthat'the amplitude of afsignal pulse of the polarity required to trigger bistable circuit 30"mustbe increased to produce an output across'resistor 21. Low amplitude pulses andnois'e ofJthe "opposite polarity are passed at lesser .amplitudesbut are ineffective to trigger v bistable circuit" 30 b ecauseof their polarity.

Immediately following a change of condition of bistable circuit 30, a minimum amplitude of signal pulse of the proper polarity will trigger bistable circuit 30 in the opposite direction. The required amplitude increases with time as condenser 29 charges through resistor 28. The noise and hips can thus increase in amplitude after each change of state without producing a premature triggering and yet the circuit is set to operate on the lower amplitude .pulse produced. if the recorded information includes. two successive changes of the type of digit.

It will be recognized that. rectifiers 25 and 26 could be replaced by a varistor as disclosed in US. patent application Serial Number 539,717 filed October 11, 1955 by Robert F. Casey; and. similar resultswould still be obtained. It will be recognized further that various other changes and modifications can be made without departing from. thespirit and scope of the invention ,as defined in the, following claims...

What is claimed is:

1; The combination of an amplifier circuit having'input, outputand control terminals and being of atype which discriminates against signal components below amplitudelevels dependent. on .the potential appliedto its control terminal; meansfor applying signals to the input terminal'of said amplifier circuit, a bistable circuit having its triggerinput connected to theoutputterminal of said amplifier circuit and. anoutput, and means connected to thee output-.of'said bistable circuit and to the control terminal of said amplifier circuit for integrating the potential appearing on the output of said bistable circuit and for impressing-said integrated potential on the control terminal of said amplifier circuit.

2. The combination accordingto claim 1 wherein said amplifier "circuit comprises a normally conducting cathode follower having a control element connected to said input terminaha normally. conducting amplifier having an anode'connected to said output terminal, a control element connected to said control terminal, and a cathode, and r'n'eansffor coupling the outputof said cathode follower to the cathode of said amplifie'r,.said.coupling .means beingof a type offering a higher impedance to potential difiei'ences below a givenmagnitude'thanto those above the given magnitude.

3. The combination according to claim 2 .wherein.said coupling means comprises first and second rectifiers hav- 'ingtheir cathodes connected'respectively to the cathodes of said cathode follower and said amplifierand a pair of resistors". connecting the anodesof said'second and first rectifiers'respective'ly withthe cathodes .of saidlcathode follower and said amplifier;

4; The combination according to claim 2 wherein said coupling means comprises a varistor.

vReferences Citedin the fileof this patent UNITED STATES PATENTS We'issman;. Apr. 28, 3 Talamini Jr. et all Dec. 1, 1953 

